Method

1. Remove the skin from the fish and cuts each fillet into 4
2. Place in a saucepan with onion and lemon juice
3. Add the water and milk
4. Cover and cook gently for 10 minutes
5. Drain the fish, saving the stock
6. Melt the margarine in a different saucepan. Add the flour and cook for 2 minutes until it forms a soft ball
7. Remove from heat and add the fish stock gradually, stirring continuously
8. Bring to the boil, reduce heat and cook for another 2 minutes
9. Grease a pie dish and add a little sauce
10. Place fish in the dish and cover with remaining sauce
11. Spoon the mashed potatoes around the edge
12. Brown under the grill or in the oven